The present invention relates to an improved method for cabling of data communications between data terminals and data sets. In particular, the present invention is a cable designed to replace conventional cables that are individualized (or customized) for a variety of male/female and/or null modem configurations necessary to meet the cabling requirements for data communications in the marketplace.
The transmission of data from a data terminal (computer) to a data set (peripheral) is essential to disseminate information whether such dissemination is to a printer for documentation of data in a permanent form or to another peripheral for further manipulation of data. The use of data transmitted is limited only by one's imagination but the conventional method of transmitting such data to a printer, modem, etc... (hereinafter peripheral device) is by means of low resistance continuous wire connected directly from the computer (source) to the peripheral (destination) device. To accommodate the transmission of data the source device must have an output terminal to match the input terminal of the destination device.
The means used to carry the data may be one wire or a bundle of wires commonly referred to as a cable between the devices. A connector on each end of the cable is a convenient means to attach the cable to the device. The computer industry has standardized the various types of connectors. An established standardized connector to provide for the input/output of data to the device is a D-type connector. This connector does not necessary utilize each pin but is a connector that allows a wide variety of cable configurations from a single connector allowing manufacturers to standardize on the input/output connection but maintain the flexibility of designing their own wiring requirements between devices.
Systems having large numbers of signals to be carried by wire may use a standardized 9-pin, 15-pin, 25-pin, 37-pin or 50-pin D-type connector. The present invention can be used for any type connector but for illustration purposes and simplicity of discussion, the commonly used D-25 pin connector will be discussed hereinafter.
The problems associated with the present art of cabling between devices begin with the transmitting device mentioned which may also be a device that receives information from another transmitting device. The cabling between devices must handle this type of input/output of signals typically called data communication for streams of digital data is transferred between the computer (source) and the peripheral (destination). For example, data communications involving a computer and a modem can be simply coordinated by two inputs DSR (data set ready) and CTS (clear to send) and two outputs DTR (data terminal ready) and RTS (request to send). The computer utilizing the DTS & CTS pins of a D-25 connector and the modem utilizing the DTA & RTS pins of the D-25 connector are connected at the opposite end of the cable. To operate, the computer (data terminal) pulls its DTR and RTS pins low indicating to the modem (data set) that the "data terminal" is ready to send data and is requesting permission to send data. The data terminal DTR is connected to the data set's DSR and the data terminal RTS is connected to the data set's CTS line. The data set also pulls its DSR and CTS pins low indicating to the computer (data terminal) that the "data set" is ready to receive data. In this example, the computer, called the data terminal, is transmitting but it should be apparent that the data terminal will also accept data, whereby it could be referred to as data set. The point is that the cabling between DSR of the data set and DST of the data terminal must be directly coupled and the CTS of the data set and RTS of the data terminal must also be directly coupled. This scheme is perfectly adequate if the only devices connected were terminal and modem requiring two wires. Connections between such devices would consist of a cable connected to "pin 1" at one end of the cable/connector and to "pin 2" at the other end of the cable/connector, and Pin 2 at the one end to Pin 1 at the other end. As the amount of pins increase for different purposes such as a RI (ring indicator) to allow a computer to answer a phone and the RLSD (received line signal detect), also called the carrier line indicating that the carrier from the modem is detected (an audio tone on which several signals sent over the phone lines are superimposed) the problem occurs of which the present invention is directed. Connections of this type could not be made directly from a terminal to a data set (pin 2--2, pin 313 3, etc.) for each device considers itself the data terminal and would transmit data to an unreceptive pin. Even the TD (transmit data) and RDC (receive data) pins have this implied functionality, since the terminal transmits data on the TD pin but the modem receives data on the TD pin and the modem transmits its data to the RD pin so that the terminal can receive on the RD pin. An attempt to resolve this situation resulted in the use of multiple pin connectors designed to prevent direct wiring, by using a DB-25S female connector coupled to the data terminal DB-25P male connector to the data set. This crossed cabling technique is also known as a "null modem" cable, since it eliminated the need for modems to be placed between two terminal devices which were connected together.
The null modem did not resolve all problems for some computer manufacturers wired their hardware as terminal only. Therefore, communication wiring to another computer would have to be with null modem cable or by wiring to a modem with straight cable. Alternatively, computer manufacturers who wired their hardware as a data set required a null modem cable for connection to another computer (if set up as a terminal output) and a straight cable to connect to a conventional modem.
In addition, a major problem with the null modem scheme on a module pin cable is demonstrated by the DCD pin (data carrier detect) or any other pin that does not have a matching pin at the receiving (or sending) device. In these situations a common practice is to float the pin, i.e., no corresponding connector, or to couple it to the DSR or another pin where it would not cause additional problems.
The net effect of the situation is that the connection of any two devices might require either a malemale straight, or a male-female straight, or a female-female straight, or a male-male null, or a male-female null, or a female-female null and such a connection would not ensure that each source pin was wired to a "correct" corresponding pin.
Further problems exist when changing a straight cable to a null modem on a D-25 connector. Such a change requires six or more wires to be altered and jumpers must be inserted or removed. This problem alone created a whole industry for null modem adapters and female-male gender changers. However, the adapters are expensive, cumbersome and unnecessary in light of the instant invention.
The present invention is directed to a new and novel way of cabling the devices together by means of a universal cable that requires only two types of connectors to create all possible combinations of male/female connections. Thus, the present invention allows a conversion from a straight cable to a null cable by simply reversing the orientation of the snap-on connector.